Beispiel #1
0
 /**
  * Spins/blocks until node s is matched by a fulfill operation.
  *
  * @param s the waiting node
  * @param timed true if timed wait
  * @param nanos timeout value
  * @return matched node, or s if cancelled
  */
 SNode awaitFulfill(SNode s, boolean timed, long nanos) {
   /*
    * When a node/thread is about to block, it sets its waiter
    * field and then rechecks state at least one more time
    * before actually parking, thus covering race vs
    * fulfiller noticing that waiter is non-null so should be
    * woken.
    *
    * When invoked by nodes that appear at the point of call
    * to be at the head of the stack, calls to park are
    * preceded by spins to avoid blocking when producers and
    * consumers are arriving very close in time.  This can
    * happen enough to bother only on multiprocessors.
    *
    * The order of checks for returning out of main loop
    * reflects fact that interrupts have precedence over
    * normal returns, which have precedence over
    * timeouts. (So, on timeout, one last check for match is
    * done before giving up.) Except that calls from untimed
    * SynchronousQueue.{poll/offer} don't check interrupts
    * and don't wait at all, so are trapped in transfer
    * method rather than calling awaitFulfill.
    */
   long lastTime = timed ? System.nanoTime() : 0;
   Thread w = Thread.currentThread();
   SNode h = head;
   int spins = (shouldSpin(s) ? (timed ? maxTimedSpins : maxUntimedSpins) : 0);
   for (; ; ) {
     if (w.isInterrupted()) s.tryCancel();
     SNode m = s.match;
     if (m != null) return m;
     if (timed) {
       long now = System.nanoTime();
       nanos -= now - lastTime;
       lastTime = now;
       if (nanos <= 0) {
         s.tryCancel();
         continue;
       }
     }
     if (spins > 0) spins = shouldSpin(s) ? (spins - 1) : 0;
     else if (s.waiter == null) s.waiter = w; // establish waiter so can park next iter
     else if (!timed) LockSupport.park(this);
     else if (nanos > spinForTimeoutThreshold) LockSupport.parkNanos(this, nanos);
   }
 }
Beispiel #2
0
 /**
  * Tries to match node s to this node, if so, waking up thread. Fulfillers call tryMatch to
  * identify their waiters. Waiters block until they have been matched.
  *
  * @param s the node to match
  * @return true if successfully matched to s
  */
 boolean tryMatch(SNode s) {
   if (match == null && UNSAFE.compareAndSwapObject(this, matchOffset, null, s)) {
     Thread w = waiter;
     if (w != null) { // waiters need at most one unpark
       waiter = null;
       LockSupport.unpark(w);
     }
     return true;
   }
   return match == s;
 }
Beispiel #3
0
 /**
  * Spins/blocks until node s is fulfilled.
  *
  * @param s the waiting node
  * @param e the comparison value for checking match
  * @param timed true if timed wait
  * @param nanos timeout value
  * @return matched item, or s if cancelled
  */
 Object awaitFulfill(QNode s, Object e, boolean timed, long nanos) {
   /* Same idea as TransferStack.awaitFulfill */
   long lastTime = timed ? System.nanoTime() : 0;
   Thread w = Thread.currentThread();
   int spins = ((head.next == s) ? (timed ? maxTimedSpins : maxUntimedSpins) : 0);
   for (; ; ) {
     if (w.isInterrupted()) s.tryCancel(e);
     Object x = s.item;
     if (x != e) return x;
     if (timed) {
       long now = System.nanoTime();
       nanos -= now - lastTime;
       lastTime = now;
       if (nanos <= 0) {
         s.tryCancel(e);
         continue;
       }
     }
     if (spins > 0) --spins;
     else if (s.waiter == null) s.waiter = w;
     else if (!timed) LockSupport.park(this);
     else if (nanos > spinForTimeoutThreshold) LockSupport.parkNanos(this, nanos);
   }
 }
Beispiel #4
0
    /** Puts or takes an item. */
    Object transfer(Object e, boolean timed, long nanos) {
      /* Basic algorithm is to loop trying to take either of
       * two actions:
       *
       * 1. If queue apparently empty or holding same-mode nodes,
       *    try to add node to queue of waiters, wait to be
       *    fulfilled (or cancelled) and return matching item.
       *
       * 2. If queue apparently contains waiting items, and this
       *    call is of complementary mode, try to fulfill by CAS'ing
       *    item field of waiting node and dequeuing it, and then
       *    returning matching item.
       *
       * In each case, along the way, check for and try to help
       * advance head and tail on behalf of other stalled/slow
       * threads.
       *
       * The loop starts off with a null check guarding against
       * seeing uninitialized head or tail values. This never
       * happens in current SynchronousQueue, but could if
       * callers held non-volatile/final ref to the
       * transferer. The check is here anyway because it places
       * null checks at top of loop, which is usually faster
       * than having them implicitly interspersed.
       */

      QNode s = null; // constructed/reused as needed
      boolean isData = (e != null);

      for (; ; ) {
        QNode t = tail;
        QNode h = head;
        if (t == null || h == null) // saw uninitialized value
        continue; // spin

        if (h == t || t.isData == isData) { // empty or same-mode
          QNode tn = t.next;
          if (t != tail) // inconsistent read
          continue;
          if (tn != null) { // lagging tail
            advanceTail(t, tn);
            continue;
          }
          if (timed && nanos <= 0) // can't wait
          return null;
          if (s == null) s = new QNode(e, isData);
          if (!t.casNext(null, s)) // failed to link in
          continue;

          advanceTail(t, s); // swing tail and wait
          Object x = awaitFulfill(s, e, timed, nanos);
          if (x == s) { // wait was cancelled
            clean(t, s);
            return null;
          }

          if (!s.isOffList()) { // not already unlinked
            advanceHead(t, s); // unlink if head
            if (x != null) // and forget fields
            s.item = s;
            s.waiter = null;
          }
          return (x != null) ? x : e;

        } else { // complementary-mode
          QNode m = h.next; // node to fulfill
          if (t != tail || m == null || h != head) continue; // inconsistent read

          Object x = m.item;
          if (isData == (x != null)
              || // m already fulfilled
              x == m
              || // m cancelled
              !m.casItem(x, e)) { // lost CAS
            advanceHead(h, m); // dequeue and retry
            continue;
          }

          advanceHead(h, m); // successfully fulfilled
          LockSupport.unpark(m.waiter);
          return (x != null) ? x : e;
        }
      }
    }